1. Field of the Invention
The present invention relates to an endoscope system for obtaining information of an oxygen saturation level of hemoglobin in blood in a blood vessel of living tissue and a method for controlling the same.
2. Description Related to the Prior Art
Examinations and acquisition of biological information, for example, blood information, using endoscopes have been common in the medical field. As is well known, the endoscope is provided with an insert section to be inserted into a body cavity of a subject. Illumination light is applied from a distal portion of the insert section to an internal body part in the body cavity. An image sensor disposed in the distal portion captures an image of the internal body part.
Conventionally, a white light source such as a xenon lamp or a metal halide lamp is used as a light source of the endoscope. Recently, a method for imaging the internal body part illuminated with narrowband light (special light) has gathered attention. This method facilitates finding a lesion (see U.S. Patent Application Publication No. 2008/0294105 corresponding to Japanese Patent No. 3583731).
A method for obtaining blood information, for example, an oxygen saturation level of hemoglobin in blood in a blood vessel, based on an image signal has been researched. The image signal is obtained while the internal body part is illuminated with the narrowband light. For example, in Japanese Patent Laid-Open Publication No. 06-315477, two or more wavelength sets of narrowband light in wavelength bands of 300 to 400 nm, around 400 nm, 400 to 500 nm, 500 to 600 nm, and 450 to 850 nm, are used. Each wavelength set includes a wavelength at which absorbance varies with the oxygen saturation level of hemoglobin and a wavelength at which the absorbance is substantially unaffected by the oxygen saturation level.
In the method disclosed in the Japanese Patent Laid-Open Publication No. 06-315477, a function having a parameter, for example, a ratio between two color pixel values, or a data table is provided as reference data in advance. The information of the oxygen saturation level is obtained from the reference data.
The reference data is based on data of typical living tissue. Actually, however, the reference data varies depending on a characteristic of an internal body part to be observed, for example, structure, thickness, and vascular density of a mucosal layer. The characteristic of the internal body part varies depending on an individual difference or a digestive organ on which the internal body part is located. Accordingly, when the oxygen saturation level of the living tissue with an atypical characteristic is calculated using the typical reference data, reliability of the calculation decreases.
The Japanese Patent Laid-Open Publication No. 06-315477 does not disclose such decrease in reliability of the calculation of the oxygen saturation level and a solution to it.